1. Field of the Invention
The present invention concerns a clutch cover assembly suitable for automotive vehicles, of the kind comprising a pressure plate, elastic means (a diaphragm spring, for example) and cover.
The invention is more particularly concerned with means for limiting axial and radial movement of the pressure plate to prevent the driving straps which provide rotational linkage and axial mobility between the pressure plate and the cover being damaged when the pressure plate/diaphragm spring/cover combination is pre-assembled but not yet bolted to the reaction plate.
2. Description of the Prior Art
It is known that, following an impact to the edge of the pre-assembled mechanism in the storage position, caused by dropping it, for example, the pressure plate is acted on by axial and radial forces causing deformation of at least two of the straps, one in bending and the other in buckling. This occurs even in the presence of means for limiting axial movement of the pressure plate.
The means in question permit substantial bending of the straps caused by axial displacement resulting from the inertia of the plate. However, the known means, such as the pins disclosed in French Patent Application No. 867 177, cannot prevent certain kinds of deformation as a consequence of radial movement of said plate.
Since in the storage position said straps are in maximum bending, further bending through only a few tenths of a millimeter or buckling through only a few hundredths of a millimeter is likely to generate stresses prejudicial to the return force properties of the straps, as they exceed their elastic limit.
Such deformation can readily occur in spite of the presence of the aforementioned pins or any other means of limiting axial movement of the pressure plate.
These pins, which are in practice rigidly attached to the pressure plate, for example, must be capable of sliding without friction in a corresponding hole in the cover during manipulation or operation of the clutch. As is well known, there is in a clutch cover assembly a lack of parallelism between the friction surface of the pressure plate and the bead through which the diaphragm spring bears on the pressure plate. This causes slantwise positioning of the pressure plate on operation of the clutch. This lack of parallelism is accentuated once the mechanism is mounted, on the one hand by the diaphragm spring which is not flat, particularly in the region of its fingers, which causes distortion of the part forming the Belleville washer at the roots of the fingers of the diaphragm spring on operation of the clutch, and on the other hand by virtue of defects in the bearing engagement between the cover and the diaphragm spring.
Moreover, when the clutch is operated (released or engaged), the cover tends to deform due to variations in the force exerted by the diaphragm spring.
For these reasons, in the prior art, such as the aforementioned patent application, for example, relatively large clearance is provided between the pin limiting axial movement of the pressure plate and the hole in the cover, so that no friction arises in operation.
This clearance is not compatible with proper radial retention of the pressure plate in the stored state.
Furthermore, if friction occurs during operation, this can have damaging consequences for the straps. In the event of an impact, the pin may come into circumferential contact with the hole in the cover which, by reaction, may cause unacceptable buckling of the straps.
The assignees of the present applicant have already proposed (in U.S. patent application Ser. No. 348,073 of Feb. 11, 1982) means for radial retention of the pressure plate.
This means consist in a lug cut out from the skirt of the cover and which is curved so that it comes into opposed relationship with a circumferential notch formed in the edge of the pressure plate.
Radial clearance is provided between the lug and the bottom of the notch, so that friction in operation is entirely eliminated.
Unfortunately, this solution does not give satisfaction in all cases, and certain disadvantages may arise in implementing it.
First of all, even if the notch can be formed during casting, it must subsequently be machined on a lathe. Thus there is in any event an additional operation in the manufacture of the pressure plate.
In any case, the shape of the lug is not simple. It is relatively short, and it is necessary to observe precise bending dimensions, all the more so in that the radial clearance margin in the mechanism is extremely tight, for the reasons mentioned above.
Also, the pressure plate is mounted in the cover by means of a bayonet coupling type movement. To this end, it is therefore necessary to provide special assembly rigs and machinery capable of carrying out an axial movement and also a rotational movement. Assembly is thus considerably complicated and the cost further increased.
The objective of the present invention is to overcome these disadvantages and to provide axial limiting and retaining means permitting friction-free sliding of the pressure plate and radial retention of said plate with all risk of deterioration of the straps avoided.